The researchers performed a review of the scientific literature to investigate the effects of hydration status on heart rate. They chose only studies that included comparisons of dehydration to euhydration (being adequately hydrated), and conducted research specifically in the heat. The search resulted in twenty studies to include in the review.

Naturally, the idea here is that when exercise is conducted in the heat, athletes lose water, mainly through sweat. This may also apply to other sports in which water is cut to achieve a specific weight class. This topic has actually come under debate, and some believe that perhaps the consequences of dehydration have been overhyped.

The researchers chose to analyze heart rate for a couple reasons. First, heart rate is a decent gauge of health, fitness, and stress. The heart responds to a variety of physical and psychological stimuli, and the way it responds has a great deal to do with a person’s wellbeing. Second, measuring heart rate is a simple test that anyone can perform and is easy for everyone to understand. Many kinds of tests require lab conditions, such as VO2 max testing, but measuring heart rate only requires a clock.

As you become dehydrated plasma volume decreases, making the blood more viscous. This in turn increases heart rate, and combined with heat the issue is exacerbated. When exposed to high temperatures, the skin requires an above-normal amount of blood, increasing the requirements on the heart rate even more.

Indeed, a substantial cardiovascular stress was discovered as a result of dehydration. Heart rate changed an average of three beats per minute for every one-percent change in bodyweight resulting from dehydration. These changes result in a reduction in performance beginning at around two-percent loss in bodyweight.

The researchers noted that losing water to the tune of one to two liters per hour is common during intense exercise, so we can easily figure out how long it would take for the average person to suffer a performance decrement. One liter of water weighs a kilogram, which is 2.2lbs. Using a fictional 150lb person, one hour of exercise in the heat (exceeding 26.5 degrees Celsius or 80 degrees Fahrenheit) is enough to lose 1.5-3% of bodyweight. In other words, enough to experience performance-dropping dehydration.

In these conditions, it wouldn't take long to find your heart rate ten beats per minute higher than normal for a similar activity. That’s more than enough to take you from an activity you can sustain indefinitely to one that exhausts you.

It’s clear from these results that, regardless of its effects on general health, the present literature agrees that dehydration negatively impacts performance. Exercising while dehydrated, which is not uncommon, can be a performance killer. Keeping hydrated and cool will mitigate this effect, so keep drinking your water.